1. Field of the Invention
The tracked cable guide assembly of this invention improves the capability for storage of electrical conductor cable in a drill pipe. Twisting is prevented or reduced in the overlapped portion of cable which is looped between upper and lower cable guides. This invention also simplifies the placement of the storage apparatus within the drill pipe.
2. Description of the Prior Art
Telemetry is a major focus for research in the drilling of oil, gas, or similar boreholes into subterranean formations. Enhanced transmission of data concerning downhole conditions could improve drilling safety and efficiency. Sending coded electrical impulses to disclose subsurface conditions is one method of telemetry. Measuring conditions and transmitting data while drilling, however, is complicated. To develop a reliable electrical transmission means has been a major goal. In conventional rotary drilling, thirty (30)-foot sections of drill pipe are added to the drill string as drilling proceeds. To maintain an electrical circuit to the surface, additional conductor cable must be available when pipe is added. Sections of cable may be added at the surface or additional cable may be stored within the drill pipe and threaded through each additional pipe section. This invention relates to an improved apparatus to store excess conductor cable within the drill pipe.
Earlier telemetry operations required that an instrument package be lowered into the drill pipe or wellbore when measurements were desired. Drilling had to stop to collect downhole data. It could not proceed until the instrument package was removed from the well. Thus, telemetry was a slow process which greatly disrupted the drilling operation.
To reduce the disruption caused by the earliest telemetry system, an electrical conductor was incorporated into the drill pipe. A circuit was formed through the pipe with special connections at each joint. U.S. Pat. Nos. 3,518,608 and 3,518,609. The incorporated conductor system was undesirable for at least three reasons. One, junctions at each joint greatly increased the circuit resistance. Greater power was required to transmit electrical impulses. Two, the junctions often short-circuited because insulating them from the drilling mud was difficult. Three, drill pipe had to be modified significantly. This modification either greatly increased the cost of telemetry, or induced operators not to use this system.
Another suggested improvement over the earliest telemetry system was to run a conductor cable inside the drill pipe from a downhole instrument package to the surface and to add additional sections of cable at the surface when additional drill pipe was connected to the drill string. In U.S. Pat. No. 2,748,358, sections of cable somewhat longer than the section of pipe added were connected to the existing conductor line. As with the modified drill pipe, however, this system proved undesirable in many circumstances. Again, the multitude of connections required increased power to transmit electrical signals. The cable connections tended to erode under the abrasive action of drilling mud. Erosion led to system failure. The excess length of cable could snarl and tangle with itself during the drilling operations, either impeding withdrawal of the cable or leading to telemetry system failure.
In U.S. Pat. No. 3,807,502, an attempt was made to reduce the twisting and snarling problem of storing excess conductor cable within the drill pipe. At the beginning of a drill bit run, a continuous length of cable was suspended from the surface to a subsurface instrument. Additional lengths of conductor cable were added for each section of pipe added during the bit run. The improved system provided a clamp to remove slack between the instrument and the surface. This system, however, was still inefficient. One, it still required a number of cable sections and connections. Two, it consumed large amounts of cable because a new length of continuous cable had to be payed out at the beginning of each bit run.
A second system to reduce twisting and snarling was disclosed in U.S. Pat. Nos. 3,825,079 and 3,918,537. Instead of a clamp to remove slack in the conductor cable within the drill pipe, these patents disclosed an apparatus to store the excess conductor cable in an overlapped configuration. A rigid track extended between an upper and a lower cable guide to permit axial motion of the lower cable guide, but to prevent relative angular movement. Typically about sixty (60) feet of excess cable could be stored in this overlapped configuration. The system contemplated adding sections of conductor cable with each, additional pipe section.
Further improvements were developed to store a continuous segment of conductor cable within the drill pipe. These improvements eliminated the multitude of connections. In one system, additional lengths of cable were withdrawn from a cable storage reservoir as pipe sections were added. U.S. Pat. Nos. 3,825,078 and 3,913,688 typically, cable was stored in an overlapped configuration between upper and lower cable guides.
In U.S. Pat. No. 4,098,342, use of a constriction in the pipe joints limited rotation of the lower cable guide. A long weight was added to the lower guide, and checks were added to each pipe joint to limit the twisting of this weight. Two design criteria of this system may limit its suitability in some circumstances. The checks have to be added at each joint and be aligned so that twisting will be prevented. The weight has to be long enough (typically about thirty-six (36) feet) so that it will continually contact at least one of the checks.
Excess conductor cable may be wrapped around a spool which is then suspended in the drill pipe. U.S. Pat. No. 4,153,120. To add pipe, additional sections of cable may be manually unwound from the spool after withdrawing the spool from the drill pipe. By necessity, this system requires slacked cable within the pipe, equal in length at least to the length of the spool plus the desired pipe length. This excess cable would tend to tangle during drilling operations.
A further development of the overlapped cable system disclosed a means to tension the segment of cable extending from the subsurface location up to the upper cable guide. U.S. Pat. No. 3,957,118. Suspending the weighted lower cable guide on a looped portion of the cable eliminated slack in the overlapped segments of the conductor cable, but did not sufficiently tension this segment of cable. The gripping means allowed a tension to be placed on this segment and maintained thereon.
This invention continues the development of the overlapped conductor cable system. It presents further improvements.